Revert "Update T3279 to use mask rather than block"

This reverts commit a6d80ce5fb2211af8dafc425705fefb316702802.
use unsafeUnmask instead of an IORef containing restore

This is much closer to how the test originally worked, so it's more
likely that the test is still testing what it was testing before :-)
small tidyup
The "unblock" was important here

Because catch implicitly masks the exception handler.